The present invention is related generally to the field of Atomic Force Microscopy (AFM), Scanning Tunneling Microscopy (STM), Near field Scanning Optical Microscopy (NSOM), NanoSpectroPhotometry (NSP), NanoPolarimetry (NP), Magnetic Field Microscopy (MFM) and any other methods adaptable and suitable to guide the scanning and nanomachining techniques described herein. These technologies are sometimes collectively referred to as Scanning probe Microscopy (SPM). Generally, SPM technologies allow one to “see” atomic-scale features on or in surfaces. More specifically, the present invention is directed to improving the collection of data in an AFM system where a 1-dimensional or 2-dimensional cantilever tip is used to scan the surface.
An AFM works by scanning a tip over a surface much the same way as a phonograph needle scans a record. The tip is located at the end of a cantilever beam and positioned over the surface to be scanned. The combination of the cantilever beam and tip is sometimes referred to collectively as a scanning probe or simply a probe.
AFM techniques rely on the effects of the inter-atomic interactions, such as van der Waals forces, that arise between the atoms in the structure of the tip and the atoms at the surface being imaged. As the tip is attracted to the surface, the cantilever beam is deflected. The magnitudes of the deflections correspond to the topological features of the atomic structure of the surface being scanned. The AFM can work with the tip touching the sample (contact mode), or the tip can tap across the surface (tapping mode), or the tip can be controlled in such a way as not to touch the surface at all (non-contact mode).
STM techniques rely on the principle that the electron probability cloud associated with the atoms at the surface extends a very small distance above the surface as described by the quantum physical model. When a tip is brought sufficiently close to such a surface, there is an increased probability of an interaction between the electron cloud on the surface and that of the tip atom. An electric tunneling current flows when a small voltage is applied. The tunneling current is very sensitive to the distance between the tip and the surface. These changes in the tunneling current with distance as the tip is scanned over the surface are used to produce an image of the surface.
The conventional method of scanning traces a path over the surface along a direction which conventionally is referred to as the X-direction (X axis). A scanning tip is adjusted Z-direction (Z axis) for each scan line, up or down relative to the surface in response to the hills and valleys present on the surface. The scanning is performed for a predetermined distance along the X-direction. The tip is then returned to roughly the initial X axis position and then indexed to the next Y-direction (Y axis) position, and the motion is repeated. This operation is commonly referred to as a raster scan and is shown in FIG. 9A by scan lines 902, 904, 906, and 908.
With conventional scanning in an AFM, the system collects the data in a single ‘slice’ of the surface. If there is a local effect (indention or extension) on the surface, it will cause a change in the information collected about the surface, depending on the number of line samples collected. For example, a surface feature can be missed if there are too few scan lines. A corollary to this observation is that a surface feature can be missed if the indexing between two scan lines is greater than a dimension of the surface feature along the indexing direction.
Consider the conventional scans of a surface 900 shown in FIGS. 9B and 9C, for example. The scan lines 902, 904, 906, and 908 are shown spaced apart by an index amount of Δy. Data is collected along the indicated scan direction. The profiles 912 illustrate a typical measurement that is collected, namely, height measurements in the Z-direction relative to a reference height, such as an area on the surface being scanned. These figures illustrate a situation where the scan line spacing is sufficiently wide as to miss a surface feature 922 (e.g., a dent in FIG. 9B and a bump in FIG. 9C). This can be especially disadvantageous during an inspection operation where the surface feature is an anomalous structure that should be detected.
The conventional solution is to perform a scans with smaller index steps. This requires that greater numbers of line samples be taken to insure that the local surface effects are included in the information about that surface. However in a scanning operation, the largest contributor of throughput is the time it takes to perform a scan and particularly the time it takes to scan in the indexing direction in preparation for the next scan. Thus, while increasing the number of scans ensures detection of surface features, this comes at the cost of greatly increased scan time.
As can be seen in FIG. 9D, a number of scan lines 910 are needed to collect up the variations on surfaces of a sidewall feature 924. This figure shows both the surfaces being scanned and a corresponding set of superimposed scan lines showing a profile of the detected variations of the surface. An analysis of this surface would require that these variations be taken into account to best detect such features, which again requires an increased number of scan lines, thus increasing the time to perform the operation.
There is a need to improve the speed of scanning while still being able to detect surface features.